The present invention relates in general to two-dimensional diode-laser arrays. The invention relates in particular to two-dimensional diode-laser array that is cylindrical in one dimension with the diode-lasers arranged such that light therefrom is directed radially inward and can be used, for example, for optically pumping a rod of a laser gain-medium concentrically located within the cylindrical array.
Several different arrangements for optically pumping a rod of a solid-state laser gain-medium such as neodymium-doped YAG (Nd:YAG) using an array of diode-lasers disposed radially around the rod has been proposed over the past two decades. In each arrangement, an important aspect is that by radially arranging lasers around the rod, optical pump energy can be symmetrically delivered to and absorbed in the rod. Preferably, in any radial position there is also a plurality of lasers extending along the rod such that the rod is also energized evenly along its length. Such an arrangement of diode-lasers may be defined as a two-dimensional array of diode-lasers xe2x80x9cwrappedxe2x80x9d around the rod, or a xe2x80x9ccylindricalxe2x80x9d two-dimensional diode-laser array. This type of cylindrical two-dimensional array is often referred to by practitioners of the art as a xe2x80x9cdoughnut arrayxe2x80x9d. The diode-lasers are typically arranged as close to the laser rod as possible, given constraints such as cooling arrangements for the rod itself. While each of the prior-art such arrangements share this same important aspect, they differ widely in the manner in which the diode lasers are assembled in the array, the manner in which they are electrically connected, and the manner in which they are cooled.
One-dimensional diode-laser arrays (longitudinal arrays) are typically formed in a common substrate having a length of about one centimeter (1 cm), a width of about one millimeter (1 mm) and a height of about one hundred micrometers (100 xcexcm). This type of array is referred to by practitioners of the art as a diode-laser bar. One such bar may contain as many as sixty individual diode-lasers (emitters). Diode-laser bars capable of delivering twenty-five watts (25 W) or more of laser light are now commercially available at cost comparable with that of a domestic light fixture. Six such bars arranged every sixty degrees of azimuth to form a cylindrical two-dimensional diode laser array around a laser rod could deliver 150 W or more of laser light to the rod. A significant problem with such a powerful array, however, is that about 150 W or more of heat would be generated in a relatively small volume, close to the rod being pumped. Accordingly, in configuring a mechanical assembly for such an array, cooling arrangements are important.
One mechanical assembly that is suitable for cooling such a high power cylindrical diode-laser laser array is disclosed in U.S. Pat. No. 5,521,936. In this assembly, a plurality of copper segments is arranged around a transparent coolant flow tube in which a laser rod to be pumped is located. Diode-laser bars are located between the copper segments. Current for powering the diode-laser bars is passed in series through the segments and the diode laser bars. Another mechanical assembly that is suitable for cooling such a high power cylindrical diode-laser laser array is disclosed in U.S. Pat. No. 5,627,850. In this arrangement, laser diode bars are located in slots cut in copper segments directly bonded to a dielectric block. The copper segments form a conductor ring around the dielectric block. In one embodiment, a coolant flow tube within the conductor ring forms an annular coolant-channel around a laser rod to be pumped.
While the above-discussed assemblies provide adequate cooling for a high power cylindrical diode-laser array, components of the assemblies appear to be somewhat complex, and assembly of the components appears to be intricate. This could lead to the cost of the mechanical support and cooling of the array being greater than the cost of the diode-lasers themselves. There is clearly a need to reduce the cost of such support and cooling assemblies.
The present invention is directed to a convenient method for assembling a cylindrical, two-dimensional array of laser diodes. In one aspect of the invention, the diode-laser array comprises a heat-sink having a circular aperture extending therethrough and defining a cylindrical inner wall of the heat-sink. A plurality of fractionally-cylindrical segments of a dielectric material are bonded to the inner wall of the heat-sink. Each of the segments has a metal layer on an inward-facing surface and longitudinal edges thereof. The longitudinal edges of the adjacent segments face each other and are spaced apart defining a gap between the segments. A plurality of longitudinally spaced-apart diode-lasers is disposed in each of the gaps, thereby forming the cylindrical two-dimensional array.
In another aspect of the present invention, a method of assembling a diode-laser array comprises providing a metal heat-sink having a circular aperture extending therethrough and defining a cylindrical inner wall of the heat-sink. A cylindrical tube of a dielectric material is provided. The tube has length greater than the length of the heat-sink aperture. A plurality of longitudinal slots is cut through a first portion of the tube wall. Each of the first longitudinal slots has a length about equal to or greater than the length of the aperture but shorter than the length of the tube. The slots divide the first portion of the tube wall into a plurality of fractionally-cylindrical segments of the dielectric material, and leave a second portion of the tube substantially intact and attached to the plurality of fractionally cylindrical segments. Each of the segments has an inner surface, an outer surface and longitudinal edges. The longitudinal edges of adjacent ones of the segments face each other and are spaced apart defining a gap therebetween. The inner surface, outer surface, and longitudinal edges of each of the segments are metallized. The metallized segments are inserted into the circular aperture of the heat-sink with the intact portion of the tube still attached to the segments and outside of the circular aperture. The metallized segments are bonded to the inner wall of the heat-sink. Following the bonding, the undivided portion of the tube is separated from the bonded segments.
The pluralities of diode-lasers are located in the gaps between segments. Preferably, each plurality of laser diodes is a one-dimensional array thereof formed on a common substrate, i.e., is in the form of a diode-laser bar.
Preferably the heat-sink is made of a metal having high thermal conductivity such as copper, aluminum, molybdenum or alloys having any of these metals as a majority constituent. The dielectric material also preferably has a high thermal conductivity. Suitable dielectric materials include beryllium oxide, aluminum oxide, and aluminum nitride.
Use of the diode-laser array of the present invention is not limited to optically pumping solid-state laser rods. Other uses include but are not limited to heat treating, melting or softening of materials in tube or rod form and welding of plastic tubing.
A particular advantage of the present invention is that there is no requirement to individually manufacture and assemble the dielectric segments. The dielectric tube can be slotted according to the number of segments required while still leaving an intact end of the tube holding the segments precisely in their required alignment while they are being bonded in the heat-sink. It is believed that this can significantly reduce the cost of manufacturing the inventive diode laser array assembly compared with prior art assemblies.
In another aspect of the present invention a cylindrical diode-laser array includes an air-cooled heat-sink unit. The air-cooled heat-sink-unit includes a cylindrical portion having an aperture extending longitudinally therethrough. The cylindrical portion of the heat-sink unit has a plurality of longitudinally-oriented fins extending radially outward therefrom. A plurality of diode-laser bars is located in the aperture and arranged to form a cylindrical two-dimensional diode-laser array cooled by the air-cooled heat-sink unit. In a preferred embodiment of the heat sink unit, the fins are surrounded by a sleeve, and a fan is arranged to move air through the sleeve and over the fins.